HIV-1 protease is the virus encoded enzyme necessary for the processing of the gag-pol-polyprotein. Inhibition of this enzyme yields nonfectious immature virions. ABT-538, a specific inhibitor of HIV protease, may prevent new rounds of retroviral infection in HIV-infected individuals. This protocol is a double blind, randomized, two arm, parallel, multicenter, multicountry, clinical trial in HIV infected patients with CD4 cell counts of <100 cells/mm3. Eligible patients will have received at least nine months of previous therapy with ZDV, ddI, ddC or d4T. This study will compare ABT-538 and placebo administered orally at a dose of 600 mg bid when added to the patient's existing nucleoside therapy (if any). About 700 patients will be enrolled, 350 per arm. The double-blind phase of the study (4 months) will terminate when 191 patients have experienced disease progression. At this time, optional, open label ABT-538 continuation will be initiated.